Conventionally, field effect transistors of MIS (metal/insulator/silicon) type have been formed on a silicon substrate having a <100> surface orientation. This was because high electric performance and high reliability, such as excellent insulator/silicon interface characteristics, excellent breakdown characteristics, or excellent leakage characteristics of oxide film, could be achieved only in the case a <100>-oriented silicon is used in a thermal oxidation process (conducted generally at 800° C. or more), which has been used conventionally for forming a gate insulation film of field effect transistors. Herein, the surface orientation is defined such that a <100> direction encompasses all the directions crystallographically equivalent to a [100] direction, such as [100] direction, [010] direction, [001] direction, and the like. Similarly, a <100> surface is used in the present invention as encompassing all the crystal surfaces crystallographically equivalent to a (100) surface, such as (100) surface, (010) surface, and (001) surface.
Conventionally, it has been known that the mobility, and hence current drivability, of a field effect transistor changes depending on the surface orientation of silicon. On the other hand, no reliable experimental knowledge has been obtained so far with regard to the surface orientation that provides maximum mobility by way of attempt of choosing a surface orientation of higher mobility by forming a gate oxide film on silicon oriented in a surface orientation other than the <100> surface, because of large interface state density formed at the oxide/silicon interface as compared with the case of the silicon oxide film formed on the silicon surface oriented in the <100> orientation, and in view of poor breakdown characteristics and poor leakage characteristics of such a gate oxide film.
Meanwhile, it has been discovered, in Japanese Laid-Open Patent Publication 2001-16055, that it is possible to form a high-quality silicon oxide film, silicon oxynitride film or a silicon nitride film, on a silicon surface of any surface orientation, including the silicon surface having a <111> surface orientation, at low temperature such as 400° C. by using microwave-excited plasma. However, this prior art is entirely silent about dependency of mobility of a field effect transistor on the silicon surface orientation. Thus, there has been no knowledge so far of forming a field effect transistor structure of high mobility by choosing surface orientation of silicon surface.